1. Technical Field
The embodiments described herein are related to the field of golf balls and, more particularly, to a spherically symmetrical golf ball having a dimple pattern that generates low-lift in order to control dispersion of the golf ball during flight.
2. Related Art
The flight path of a golf ball is determined by many factors. Several of the factors can be controlled to some extent by the golfer, such as the ball's velocity, launch angle, spin rate, and spin axis. Other factors are controlled by the design of the ball, including the ball's weight, size, materials of construction, and aerodynamic properties.
The aerodynamic force acting on a golf ball during flight can be broken down into three separate force vectors: Lift, Drag, and Gravity. The lift force vector acts in the direction determined by the cross product of the spin vector and the velocity vector. The drag force vector acts in the direction opposite of the velocity vector. More specifically, the aerodynamic properties of a golf ball are characterized by its lift and drag coefficients as a function of the Reynolds Number (Re) and the Dimensionless Spin Parameter (DSP). The Reynolds Number is a dimensionless quantity that quantifies the ratio of the inertial to viscous forces acting on the golf ball as it flies through the air. The Dimensionless Spin Parameter is the ratio of the golf ball's rotational surface speed to its speed through the air.
Since the 1990's, in order to achieve greater distances, a lot of golf ball development has been directed toward developing golf balls that exhibit improved distance through lower drag under conditions that would apply to, e.g., a driver shot immediately after club impact as well as relatively high lift under conditions that would apply to the latter portion of, e.g., a driver shot as the ball is descending towards the ground. A lot of this development was enabled by new measurement devices that could more accurately and efficiently measure golf ball spin, launch angle, and velocity immediately after club impact.
Today the lift and drag coefficients of a golf ball can be measured using several different methods including an Indoor Test Range such as the one at the USGA Test Center in Far Hills, N.J., or an outdoor system such as the Trackman Net System made by Interactive Sports Group in Denmark. The testing, measurements, and reporting of lift and drag coefficients for conventional golf balls has generally focused on the golf ball spin and velocity conditions for a well hit straight driver shot—approximately 3,000 rpm or less and an initial ball velocity that results from a driver club head velocity of approximately 80-100 mph.
For right-handed golfers, particularly higher handicap golfers, a major problem is the tendency to “slice” the ball. The unintended slice shot penalizes the golfer in two ways: 1) it causes the ball to deviate to the right of the intended flight path and 2) it can reduce the overall shot distance.
A sliced golf ball moves to the right because the ball's spin axis is tilted to the right. The lift force by definition is orthogonal to the spin axis and thus for a sliced golf ball the lift force is pointed to the right.
The spin-axis of a golf ball is the axis about which the ball spins and is usually orthogonal to the direction that the golf ball takes in flight. If a golf ball's spin axis is 0 degrees, i.e., a horizontal spin axis causing pure backspin, the ball will not hook or slice and a higher lift force combined with a 0-degree spin axis will only make the ball fly higher. However, when a ball is hit in such a way as to impart a spin axis that is more than 0 degrees, it hooks, and it slices with a spin axis that is less than 0 degrees. It is the tilt of the spin axis that directs the lift force in the left or right direction, causing the ball to hook or slice. The distance the ball unintentionally flies to the right or left is called Carry Dispersion. A lower flying golf ball, i.e., having a lower lift, is a strong indicator of a ball that will have lower Carry Dispersion.
The amount of lift force directed in the hook or slice direction is equal to: Lift Force*Sine(spin axis angle). The amount of lift force directed towards achieving height is: Lift Force*Cosine(spin axis angle).
A common cause of a sliced shot is the striking of the ball with an open clubface. In this case, the opening of the clubface also increases the effective loft of the club and thus increases the total spin of the ball. With all other factors held constant, a higher ball spin rate will in general produce a higher lift force and this is why a slice shot will often have a higher trajectory than a straight or hook shot.
Table 1 shows the total ball spin rates generated by a golfer with club head speeds ranging from approximately 85-105 mph using a 10.5 degree driver and hitting a variety of prototype golf balls and commercially available golf balls that are considered to be low and normal spin golf balls:
TABLE 1Spin Axis, degreeTypical Total Spin, rpmType Shot−302,500-5,000Strong Slice−151,700-5,000Slice01,400-2,800Straight+151,200-2,500Hook+301,000-1,800Strong Hook
If the club path at the point of impact is “outside-in” and the clubface is square to the target, a slice shot will still result, but the total spin rate will be generally lower than a slice shot hit with the open clubface. In general, the total ball spin will increase as the club head velocity increases.
In order to overcome the drawbacks of a slice, some golf ball manufacturers have modified how they construct a golf ball, mostly in ways that tend to lower the ball's spin rate. Some of these modifications include: 1) using a hard cover material on a two-piece golf ball, 2) constructing multi-piece balls with hard boundary layers and relatively soft thin covers in order to lower driver spin rate and preserve high spin rates on short irons, 3) moving more weight towards the outer layers of the golf ball thereby increasing the moment of inertia of the golf ball, and 4) using a cover that is constructed or treated in such a ways so as to have a more slippery surface.
Others have tried to overcome the drawbacks of a slice shot by creating golf balls where the weight is distributed inside the ball in such a way as to create a preferred axis of rotation.
Still others have resorted to creating asymmetric dimple patterns in order to affect the flight of the golf ball and reduce the drawbacks of a slice shot. One such example was the Polara™ golf ball with its dimple pattern that was designed with different type dimples in the polar and equatorial regions of the ball.
In reaction to the introduction of the Polara golf ball, which was intentionally manufactured with an asymmetric dimple pattern, the USGA created the “Symmetry Rule”. As a result, all golf balls not conforming to the USGA Symmetry Rule are judged to be non-conforming to the USGA Rules of Golf and are thus not allowed to be used in USGA sanctioned golf competitions.
These golf balls with asymmetric dimples patterns or with manipulated weight distributions may be effective in reducing dispersion caused by a slice shot, but they also have their limitations, most notably the fact that they do not conform with the USGA Rules of Golf and that these balls must be oriented a certain way prior to club impact in order to display their maximum effectiveness.
The method of using a hard cover material or hard boundary layer material or slippery cover will reduce to a small extent the dispersion caused by a slice shot, but often does so at the expense of other desirable properties such as the ball spin rate off of short irons or the higher cost required to produce a multi-piece ball.